Mitigation of fouling in bitumen furnaces by pigging

Processing of Athabasca oil sands bitumen requires preheating of the feed t o temperatures in the range of 350-500 degrees C. At these temperatures sig nificant amounts of solid foulant can be deposited on the walls of the furn aces. Ultimately, buildup of the deposit causes premature shutdown of the p rocess, either because of excessively high tube skin temperatures or becaus e of high pressure drop across the furnace. Steam/air de-coking, which has been practiced in the past, is unable to eliminate all the deposit since th e foulant contains significant quantities of non-carbonaceous material. Cur rently, the preferred method to address this problem is pigging assisted by high-pressure water. The feed to the furnaces typically contains close to 0.5 wt % inorganic solids originating from the mineral matrix from which th e bitumen is extracted. Elemental analysis of the feed identifies 300-450 p pm iron, much of which can be attributed to the components of the inorganic matrix, such as pyrite, iron carbonate, and iron-bearing clays. However, i t is expected that soluble organic iron species, such as naphthenates and c arboxylates, are more likely to decompose at the temperatures in the furnac es and to convert to iron sulfide and deposit within the furnace. Samples o f foulant were collected from two furnace pigging operations, one at the Su ncor Energy Inc. coker furnace and one at the Syncrude Canada Ltd. LC Finer furnace. The samples were taken at timed intervals that allowed a profile of the foulant within the cross-section of the furnace tube to be completed . After washing and drying, the foulant samples were analyzed for carbon, s ulfur, and a variety of metals and inspected by optical petrography. Coker furnace foulants exhibited high concentrations of both carbonaceous materia l and iron sulfide. The iron sulfide concentration was highest at the wall and declined as the distance from the wall increased. The carbon content sh owed the reverse trend, suggesting that coke formation became the more impo rtant factor as the deposition process proceeded. This reverse trend coinci ded with the increase in tube skin temperature. Fluid and skin temperatures are much lower in the LC Finer furnace than in the coker furnace; therefor e, the tendency to coke formation is reduced. As a result, the composition of the foulant from the LC Finer furnace was more constant and consisted al most entirely of iron sulfide. It was estimated that the amount of iron dep osited as foulant corresponded to about 0.05 wt % of the total iron in the feed that passed through these furnaces.